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I64ToI32Lowering: Document non-trapping float truncation behavior. NFC #9022

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Merged
sbc100 merged 1 commit into from
Aug 19, 2026
+25 −8
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I64ToI32Lowering: Document non-trapping float truncation behavior. NFC #9022

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33 changes: 25 additions & 8 deletions src/passes/I64ToI32Lowering.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -15,10 +15,17 @@
*/

//
// Lowers i64s to i32s by splitting variables and arguments
// into pairs of i32s. i64 return values are lowered by
// returning the low half and storing the high half into a
// global.
// Lowers i64s to i32s by splitting variables and arguments into pairs of i32s.
// i64 return values are lowered by returning the low half and storing the high
// half into a global.
//
// Note: This pass is designed primarily as an internal part of the wasm2js
// pipeline rather than a general-purpose lowering pass for standard WebAssembly
// runtimes. As such, it does not strictly preserve all WebAssembly trapping
// semantics. In particular, float-to-int conversions are lowered using float
// arithmetic and 32-bit truncations that do not trap on out-of-range values,
// NaN, or infinity, relying on the fact that wasm2js maps 32-bit truncations
// to non-trapping JavaScript bitwise operations (~~ and >>>).
//

#include "abi/js.h"
Expand Down Expand Up @@ -623,8 +630,8 @@ struct I64ToI32Lowering : public WalkerPass<PostWalker<I64ToI32Lowering>> {
}

void lowerReinterpretFloat64(Unary* curr) {
// Assume that the wasm file assumes the address 0 is invalid and roundtrip
// our f64 through memory at address 0
// Roundtrip the f64 through a scratch buffer via wasm2js helper functions
// to extract the low and high 32-bit integer halves.
TempVar highBits = getTemp();
Block* result = builder->blockify(
builder->makeCall(
Expand All @@ -643,8 +650,8 @@ struct I64ToI32Lowering : public WalkerPass<PostWalker<I64ToI32Lowering>> {
}

void lowerReinterpretInt64(Unary* curr) {
// Assume that the wasm file assumes the address 0 is invalid and roundtrip
// our i64 through memory at address 0
// Roundtrip the low and high 32-bit integer halves through a scratch buffer
// via wasm2js helper functions to reconstruct the f64 value.
TempVar highBits = fetchOutParam(curr->value);
Block* result = builder->blockify(
builder->makeCall(ABI::wasm2js::SCRATCH_STORE_I32,
Expand All @@ -661,6 +668,16 @@ struct I64ToI32Lowering : public WalkerPass<PostWalker<I64ToI32Lowering>> {
}

void lowerTruncFloatToInt(Unary* curr) {
// Lowers 64-bit float-to-int truncations into 32-bit float arithmetic and
// 32-bit truncations.
//
// Note that this lowering is non-trapping: in wasm2js, the emitted
// 32-bit truncations are translated to JavaScript bitwise operations
// ((~~expr) >>> 0), so out-of-range values, NaN, and +/-infinity do not
// trap. Both signed and unsigned operations share the same logic because
// the two's complement bit representation is identical for in-range values.
//
// Pseudocode:
// hiBits = if abs(f) >= 1.0 {
// if f > 0.0 {
// (unsigned) min(
Expand Down
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